1. Field of the Invention
The present invention is generally related to an outboard motor with a supercharger and, more particularly, to an outboard motor having a screw compressor which provides pressurized charge air for the engine.
2. Description of the Prior Art
Superchargers used in conjunction with internal combustion engines are well known to those skilled in the art. In automobile applications, both Roots compressors and screw compressors have been used to provide pressurized charge air for the engine. U.S. Pat. No. 5,332,376, which issued to Lindbrandt on Jul. 26, 1994, describes a screw compressor for use with an internal combustion engine. The screw compressor, especially adapted for use as a supercharger for an internal combustion engine, comprises a housing having a barrel section preferably made from aluminum and two end sections, in which housing a pair of screw rotors are mounted. The end sections are provided with projections inserted into the end portions of the barrel section by forced fit between surfaces of the projections. The end portions of the barrel section are also secured to the barrel section by screw joints.
U.S. Pat. No. 5,241,817, which issued to George, Jr. on Sep. 7, 1993, describes a screw compressor with regenerative braking. A screw engine for propelling a wheeled vehicle via a continuously variable transmission is characterized by a ceramic screw compressor and a ceramic screw expander mounted on a common shaft of the continuously variable transmission. The compressor compresses air and delivers it to a combustion chamber. The air is mixed with fuel in the chamber and ignited to produce combustion products. The combustion products are expanded by the expander to produce a work output for driving the continuously variable transmission and an exhaust output. A heat exchanger is connected with the expander to recover heat from the exhaust output.
U.S. Pat. No. 4,802,457, which issued to Oscarrson on Feb. 7, 1989, describes an internal combustion engine provided with a supercharger. In a conventional throttle-controlled internal combustion engine equipped with a supercharger, a compressor is provided with screw rotors arranged in a compression chamber. The supercharger is inactive at part engine loads. Since the engine normally operates at part loads for about 95% of its running time, efficiency is correspondingly poor. This drawback is overcome with the arrangement according to the invention in which a screw compression is provided on the inlet side thereof with a capacity regulating device which is opened by a gas pedal, or accelerator, and which when the engine is only partially loaded, is adjusted to a corresponding position in which the compressor operates as an expander of variable throttle effect on the engine inlet side and transmits power to the engine, thereby replacing the conventional gas throttle.
When superchargers are used in conjunction with an internal combustion engine, they are typically used to provide additional power at high loads and engine speeds. In other words, at high engine speeds, the supercharger provides additional pressurized charge air to increase the horsepower output of the engine to a magnitude greater than would than otherwise be achievable without the use of a supercharger. At lower engine speeds, the supercharger is typically either deactivated completely or its overall effect on engine operation is reduced by bypassing a significant percentage of the pressurized air emitted by the compressor.
In certain types of outboard motor applications, such as with four cycle internal combustion engines, the power output of the engine at low engine operating speeds is insufficient for the intended operation of the outboard motor. For example, when an outboard motor is used to propel a marine vessel, certain power requirements are needed to increase the speed of the marine vessel from a stationary condition to a sufficient velocity that enables the vessel to rise up to a planing condition. This requires the generation of significant power at relatively low engine speeds. If the engine is sized to provide this level of output power at low engine operating speeds, it is typical and expected that the output power of the engine will exceed allowable horsepower output limits at higher engine operating speeds. It would therefore by significantly beneficial if an engine can be sized to achieve allowable power output at maximum operating speeds, but also to achieve higher than expected power output at lower engine speeds.
An outboard motor made in accordance with the preferred embodiment of the present invention comprises an internal combustion engine having a plurality of cylinders, each of said plurality of cylinders having one of a plurality of pistons slidably disposed therein for reciprocal movement. It also comprises a crankshaft that is driven by the plurality of pistons of the engine and supported within the engine for rotation about a vertical axis. A propulsor is connected in torque transmitting relation with the crankshaft. The propulsor can be a marine propeller or an impeller. An air conduit is connected in fluid communication with the plurality of cylinders and a screw compressor, having an inlet passage and an outlet passage, is connected with its outlet passage in fluid communication with the air conduit. In this way, the screw compressor provides compressed air from its outlet passage into the air conduit, or intake manifold, for transmission to each of the plurality of cylinders. More particularly, the pressurized air from the screw compressor outlet passage is provided to the combustion chambers of the plurality of cylinders.
The screw compressor comprises a first screw rotor rotatable about a first rotor axis and a second screw rotor rotatable about a second rotor axis. The first and second rotor axes are parallel to the vertical axis about which the crankshaft of the engine rotates. The outboard motor, in certain embodiments of the present invention, can further comprise a charge air cooler disposed in fluid communication with the air conduit. The outboard motor can also comprise a bypass conduit disposed in fluid communication between the inlet passage and the outlet passage of the screw compressor. A bypass valve is disposed within the bypass conduit in order to control the flow of air from the outlet passage to the inlet passage. This flow of air can be intentionally directed around the compressor as a bypass flow which allows an engine control module to regulate the amount of charge air provided by the screw compressor to the combustion chambers of the engine. Throughout the description of the present invention, the screw compressor will be described as having parallel rotor axes. This is generally true during operation of the compressor but, as is known by those skilled in the art of screw compressor design, the two rotational axes of the rotors are actually slightly closer together at the exhaust end than at the inlet end. This is done to allow for the normal expansion of the exhaust end components during operation. This slight difference is included in the definition of term xe2x80x9cparallelxe2x80x9d in this context.
A particularly preferred embodiment of the present invention further comprises a tachometer associated with the internal combustion engine which provides an engine speed signal representative of the speed of the engine. The engine control module is connected in signal communication with the tachometer and with the bypass valve. The engine control module is configured to open the bypass valve to a magnitude which is a function of the engine speed signal. More specifically, the engine control module causes the bypass valve to be closed when the engine speed signal represents an engine speed below a preselected threshold. In a preferred embodiment of the present invention, the engine control module is provided with a data matrix which contains specific states of the bypass valve for specific combinations of values that are a function of throttle position, engine speed, and manifold absolute pressure values. These relationships are determined for each particular application of the present invention. This operation reduces the bypass flow and increases the pressurized air flow to the combustion chambers. The engine control module also causes the bypass valve to be at least partially opened when the engine speed signal represents an engine speed above the preselected threshold. In this way, the engine control module reduces the amount of air that would otherwise have been provided by the screw compressor to the combustion chambers of the engine. The bypass valve is connected between the compressor""s inlet and outlet to allow recirculation of compressed air at a rate determined by the status of the bypass valve.
In certain embodiments of the present invention, the screw compressor is attached to the internal combustion engine at a front side of the engine and the first and second screw rotors have five and three flutes, respectively. The screw compressor is connected in torque transmitting relation with the crankshaft of the engine by an arrangement of two pulleys and an interconnecting belt.